The protein products of proto-oncogenes possess functions (e.g., enzymatic activity, nucleic acid binding activity) that are believed to play a role in the regulation of normal cellular growth and differentiation. Thus, analysis of the functional state of proto-oncogene-encoded proteins in human malignancies represents one experimental approach that may provide insights into the biochemical alterations within cells that contribute to oncogenic transformation. While the biochemical functions of most proto-oncogene products are not known, several have been shown to be tyrosine-specific protein kinases. Of these proto- oncogene-encoded tyrosine kinases, the most extensively characterized is the product of the c-src gene, pp60c-src. This protein is the normal cellular homolog of the Rous sarcoma virus oncogene, v-src. The transforming potential of pp60v-src and mutated species of pp60c-src appears to be related to elevations in the specific activity of the v-src- and c-src-encoded protein kinases. We have determined, in a variety of human tumor cell lines, human tumor and normal human tissues, the activity and abundance of pp60c-src. Our results show that while pp60c-src protein kinase activity is low in most types of human tumors, significant elevation of pp60c-src kinase activity can be found in all human tumors of neural origin, several sarcomas, all human breast and all colon carcinomas tested.